Methods for the production of molded plastic articles, which are useful for example for the manufacture of automotive body panels, are well known in the art. Typically, such molded plastic automotive components may be manufactured by reactive molding processes, such as for example "reaction injection molding" (RIM) in which a polyisocyanate, a polyol and a chain extender are brought together in a single operation (the polyol and chain extender may be preblended if desired) and immediately injected into a mold cavity. Other polymeric materials, such as for example polyesters, epoxies, and polyamides, may likewise be utilized in RIM-type reactive molding processes, and other molding processes, such as for example bulk molding, may be used to form the molded plastic articles. The highly reactive liquid starting materials (polymeric precursors) are injected into the mold, after having been mixed in what is commonly known as a "positively controlled mixing head".
Reaction injection molded polyurethanes are disclosed in British Pat. No. 1,534,258 and German Auslegeschrift Pat. No. 1,196,864. U S. Pat. No. 3,655,597 discloses reacting polyisocyanate, polyol, blowing agent, diamine and catalyst for the manufacture of molded polyurethane components. A detailed description of the RIM process may be found in Prepelka and Wharton "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, vol. II, Nov. 2, 1975. The RIM process also has been advantageously employed in the so called "encapsulated automotive glazing" manufacturing process, wherein a polyurethane gasket is formed around the perimeter of a glazing unit of transparent material such as glass. See U.S. Pat. No. 4,561,625. Polyester resins and epoxy resins useful for reactive molding processes are disclosed in U.S. Pat. Nos. 4,405,538 and 4,581,339, respectively. Reactive molding processes employing polyamides are generally disclosed in the "Kirk-Othmer Concise Encyclopedia of Chemical Technology", John Wiley & Sons, New York, pp. 915-920 (1985).
The reactive molding process may suitably employ any flowable polymeric precursor materials which may be injected into a mold cavity where the polymeric materials react in situ to prepare the desired plastic article. Polyurethane precursors are typically used for the production of rigid automotive components such as bumpers, and for the production of flexible body parts generally known in the automotive industry as "soft face elements".
As is commonly known, molded plastic articles may be easily removed from mold cavities through the use of release agents, such as for example soaps, waxes, silicones, and polytetrafluoroethylene. Conventional release agents generally comprise a release film or coating which is sprayed into the mold cavity prior to the molding operation, or a film which is applied before the injection process and which thereafter remains adhered to the molded plastic article. Cycle times are increased due to the necessity of reapplying the release agent after each molding cycle, or at least periodically. Also known in the art are internal release agents which are injected along with the polymeric materials into the mold cavity.
Release agents sprayed into a mold cavity typically result in a residue on either or both surfaces of the molded plastic article and the mold cavity wall. Such release agents have the disadvantage that they must be removed completely from the surfaces of the molded plastic article after its removal from the mold, since otherwise it is not possible for paint or other surface treatments to properly adhere. The additional operation of removing residual release agent from the surfaces of the molded plastic article requires additional processing costs. In cases where the release agent is not water soluble, the surfaces of the molded plastic article must be degreased or treated with solvents which detrimentally alter the finish of the molded plastic article. Another disadvantage with spray-applied release agents is that residual amounts remain in the mold cavity and tend to grow in thickness with continuous production, thereby requiring removal from time to time. The use of solvents for removing release agents from the mold cavity after each injection also lengthens cycle times, because the solvent must be allowed to evaporate after its application. U.S Pat. Nos. 4,312,672 and 3,856,908 disclose release agents for polyurethane molding operations which are sprayed or brushed into the mold cavity.
U.S. Pat. No. 4,220,727 discloses a method of producing polyurethane molded plastic articles, utilizing an internal mold release additive which is contained in the polymeric precursors when injected into the mold cavity.
U.S. Pat. No. 3,331,904 discloses a parting membrane which is placed between the mold halves and conformed to the mold cavity wall upon injection of a polymeric material. The parting membrane adheres to the molded plastic article as a coating, and is withdrawn, along with the molded plastic article, when the mold halves are parted.
U.S. Pat. No. 3,887,579 discloses a polyethylene terephthalate film which is coated on one surface with a release agent, and thereafter placed into a mold cavity with the surface having the release agent thereon exposed to the subsequently injected polymeric material. The polyethylene terephthalate film may be reused a number of times for consecutive injection molding cycles, but the surface of the film which contacts the polymeric material must be recoated with the release agent each time the mold halves are parted and the molded plastic article is ejected.
Finally, U.S. Pat. No. 3,844,523 discloses a multilayer planar release sheet which is inserted as a rigid structure into the mold cavity prior to the injection of a polymeric material which generates gas upon curing. The release sheet allows the passage of the generated gas therethrough, thereby maximizing the expansion and degassing of the curing polymeric material. Although the release sheet is reusable, it is a planar rigid structure which is designed so as not to "sag" into or conform to the mold cavity, and additionally is not retained in the mold cavity upon the parting of the mold halves.
It must be noted that the prior art referred to hereinabove has been collected and reviewed only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.